1. Field of the Invention
The present invention relates in general to a speed-ratio control apparatus arranged to control a speed ratio of a vehicular continuously variable transmission such that an actual value of an input speed of the transmission coincide with a target value of the input speed, and more particularly to setting of the target input speed value upon requirement for acceleration of a vehicle.
2. Discussion of Related Art
For a continuously variable transmission provided on a motor vehicle, there is known a speed-ratio control apparatus arranged to control the speed ratio of the continuously variable transmission such that an actual value of the input speed of the transmission coincides with a target value of the input speed, which is set on the basis of a running condition of the vehicle as represented by its running speed and an operating amount of an accelerator pedal.
JP-2001-330143A discloses an example of such a speed-ratio control apparatus for a vehicular continuously variable transmission. This speed-ratio control apparatus is arranged to set a transient target value of the input speed of the transmission, so as to change along a curve of a first-order time lag, during a normal running of the vehicle, and to feedback-control the speed ratio of the continuously variable transmission such that the actual input speed first coincides with the transient target value, and eventually coincides with a final target value, so that the speed ratio of the transmission is changed at a shifting rate that does not cause a shifting shock or a delayed shifting of the transmission. Upon requirement for rapid acceleration of the vehicle with a relatively large amount of operation of the accelerator pedal, on the other hand, the speed ratio-control apparatus implements, during a rapid shifting period, a stepping increase of the transient target input speed to a value lower than the final target value, so that the speed ratio of the transmission is changed at a relatively high rate to meet the vehicle operator's requirement for rapidly accelerating the vehicle. During a constant-rate shifting period which follows the rapid shifting period during which a difference between the actual and transient target input speeds has been reduced to a certain amount by the feedback control of the speed ratio of the transmission, the speed-ratio control apparatus gradually increases, for example, linearly increases the transient target input speed toward the final target value, for thereby further reducing the above-indicated input speed difference, so that the speed ratio is changed at a relatively low rate, to prevent the shifting shock at the end of the shifting action. The speed ratio of the continuously variable transmission is feedback-controlled such that the actual input speed initially coincides with the thus set transient target value, and then with the final target value.
The stepping increase of the transient target value of the input speed upon requirement for rapid acceleration of the vehicle with a rapid depression of the accelerator pedal by a large amount for running the vehicle with a high degree of drivability results in rapid acceleration of the vehicle, which meets the vehicle operator's desire for powerful driving of the vehicle. Where the acceleration value of the vehicle as desired by the vehicle operator is not so high, the stepping increase of the transient target input speed which results in the rapid acceleration of the vehicle unfavorably has a risk of deterioration of driving comfort as felt by the vehicle operator.